On this page we'll keep track of the Python types, functions, and operators that we've covered in class. You should also review the online Python References for more complete coverage.
| Mathematical Operators | Symbol | Meaning | Example |
|---|---|---|
| + | addition | 4 + 5 = 9 |
| - | subtraction | 9 - 5 = 4 |
| * | multiplication | 3*5 = 15 |
| / | division | 6/3 = 2 6/4 = 1 6.0/4 = 1.5 |
| % | mod/remainder | 5 % 3 = 2 |
| ** | exponentiation | 3**2 = 9, 2**3 = 8 |
| String Operators | + | concatenation | "ab"+"cd"="abcd" |
| * | repeat | "xo"*3 = "xoxoxo" |
| Comparison Operators | == | is equal to | 3 == 3 is True |
| != | is not equal to | 3 != 3 is False |
| >= | is greater than or equal to | 4 >= 3 is True |
| <= | is less than or equal to | 4 <= 3 is False |
| > | is strictly greater than | 4 > 3 is True |
| < | is strictly less than | 3 < 3 is False |
| Boolean Operators | ||
x=5 | not | flips/negates the value of a bool | (not x == 5) is False |
| and | returns True only if both parts of it are True | (x > 3 and x < 7) is True (x > 3 and x > 7) is False |
| or | returns True if at least one part of it is True | (x < 3 or x > 7) is False (x < 3 or x < 7) is True |
| Type Conversion Functions | ||
| int(x) | turn x into an integer value | int("123") == 123 |
| int can fail, e.g., int("abc") raises an error | ||
| float(x) | turn x into an float value | float("2.46") == 2.46 |
| float can fail, e.g., float("abc") raises an error | ||
| str(x) | turn x into a string value | str(432) == "432" |
| type(x) | the type of x | type(1) ==
int type(1.2) == float |
| String Functions | ||
s="colorful" | Name | Returns | Example |
| .find(str) | index of first occurrence | s.find("o") == 1 |
| s.find("e") == -1 | ||
| .rfind(str) | index of first occurrence from right end of string | s.rfind("o") == 3 |
| .index(str) | same as .find(str), error if str not in string | |
| .count(str) | number of occurrences | s.count("o") == 2 s.count("r") == 1 s.count("e") == 0 |
| .strip() | copy with leading/trailing whitespace removed | " big ".strip() == "big" |
| .split() | list of "words" in s | "big bad dog".split() == ["big","bad", "dog"] |
| .split(",") | list of "items " in s that are separated
by a comma
In general can split on any string, not just a comma, e.g., s.split(":") will split on a colon and s.split("gat") will split on the string "gat". | "this,old,man".split(",") == ["this", "old", "man"] |
| ' '.join(lst) | concatenate elements of lst, a list of strings, separated by ' ' or any string | ':'.join(['a','b','c']) == "a:b:c" | .startswith(str) | boolean if starts with string | s.startswith("color") == True s.startswith("cool") == False |
| .upper() | uppercase of s | s.upper() == "COLORFUL" |
| .lower() | lowercase of s | "HELLO".lower() == "hello" |
| .isupper() | boolean is uppercase | 'A'.isupper() == True 'a'.isupper() == False |
| .islower() | boolean is lowercase | 'A'.islower() == False 'a'.islower() == True |
| .isalpha() | boolean is alphabetic character |
'3'.isalpha() == False '?'.isalpha() == False 'z'.isalpha() == True |
| .capitalize() | capitalized s | s.capitalize() == "Colorful" |
| Miscellaneous Functions | ||
| help(x) | documentation for module x | |
| len(x) | length of sequence x, e.g., String/List | len("duke") == 4 |
| sorted(x) | return list that is sorted version of sequence/iterable x, doesn't change x | sorted("cat") == ['a','c','t'] |
| range(x) | a list of integers starting at 0 and going up to but not including x | range(5) == [0, 1, 2, 3, 4] |
| range(start, stop) | a list of integers starting at start and going up to but not including stop | range(3, 7) == [3, 4, 5, 6] |
| range(start, stop, inc) | a list of integers starting at start and going up to but not including stop with increment inc | range(3, 9, 2) == [3, 5, 7] |
| enumerate(lst) | use in a for loop, gives you (index,item) tuples for each item in a list | for (ind,item) in enumerate(lst): |
| min(x, y, z) | minimum value of all arguments | min(3, 1, 2) == 1
min("z", "b", "a") == "a" |
| max(x, y, z) | maximum value of all arguments | max(3, 1, 2) == 3
max("z", "b", "a") == "z" |
| abs(x) | absolute value of the int or float x | abs(-33) == 33 abs(-33.5) == 33.5 |
| List Functions | sum(lst) | returns sum of elements in list lst | sum([1,2,4]) == 7 |
| max(lst) | returns maximal element in lst | max([5,3,1,7,2]) == 7 |
| list(s) | returns a list of the elements from set s | |
| list(d) | returns a list of the keys from dictionary d | |
| lst.append(...) | append an element to lst, changing lst | [1,2,3].append(8) == [1,2,3,8] |
| lst.insert(pos,elt) | append elt to lst at position pos, changing lst
| [1,2,3].insert(1,8) == [1,8,2,3] |
| lst.extend(lst2) | append every element of lst2 to lst | [1,2,3].extend([8,9]) == [1,2,3,8,9] |
| lst.remove(elt) | remove first occurence of elt from lst
| [1,2,3,2,3,2].remove(2) == [1,3,2,3,2] |
| lst.sort() | sorts the elements of lst | |
| lst.index(elt) | return index of elt in lst, error if elt not in lst | [1,5,3,8].index(5) == 1 |
| lst.count(elt) | return number of occurrences of elt in lst
| [1,2,1,2,3].count(1) == 2 |
| lst.pop() | remove and return last element in
lst, so has side-effect of altering list and returns
value.
| |
| lst.pop(index) | remove and return element at position index in
lst, so has side-effect of altering list and returns
value. Default index is last value.
| |
| Math Functions (import math) | ||
| math.pi | 3.1415926535897931 | |
| math.sqrt(num) | returns square root of num as float | math.sqrt(9) == 3.0 | File Functions |
| open("filename") | opens a file, returns file object | f = open("foo.txt") |
| open("filename",mode) | specify mode of 'r', 'a', 'w', return file object | f = open("foo.txt", "a") |
| f.read() | returns a string, entire file | s = f.read() |
| Random Functions (import random) | ||
| random.choice(list_of_choices) | returns a random element from list_of_choices. Gives an error if list_of_choices has length 0. | |
| random.randint(start, end) | Returns a random integer between start and end. Unlike range() and list slicing, the largest value it can return is end, not end-1. | |
| random.random() | Returns a random float between 0 and 1. | |
| Set Functions | ||
| set(lst) | returns a set of the elements from list lst | |
| s.add(item) | adds the item into the set, and returns nothing. | |
| s.update(lst) | adds the elements in the list lst into the set, and returns nothing.
| |
| s.remove(item) | removes the item from the set, error if item not there. | |
| s.union(t) | returns new set representing s UNION t, i.e., all elements in either s OR t, t can be any iterable (e.g., a list) | |
| s.intersection(t) | returns new set representing s INTERSECT t, i.e., only elements in both s AND t, t can be any iterable (e.g., a list) | |
| s.difference(t) | returns new set representing s difference t, i.e., elements in s that are not in t | |
| s.symetric_difference(t) | returns new set representing elements in s or t, but not in both | |
| s | t | returns/evaluates to union of s and t, both must be sets. | |
| s & t | returns/evaluates to intersection of s and t, both must be sets. | |
| s - t | returns/evaluates to set with all elements in s that are not in t | |
| s ^ t | returns/evaluates to set with all elements from s and t that are not in both s and t | |
| Dictionary Functions | d[key] |
returns the value associated with key, error
if key not in dictionary d
|
| d.get(key) |
returns value associated with key,
error if key not in dictionary d
| |
| d.get(key,default) |
returns value associated with key,
returns default if key
not in d
| |
| d.keys() | returns a list/view of the keys in dictionary | |
| d.values() | returns a list/view of values in dictionary | |
| d.items() | returns a list/view of tuples, (key,item) pairs from dictionary | |
| d.iteritems() | Use only with for loop, gives (key,item) tuple pairs from the dictionary | |
| d.update(dict) |
updates the dictionary with another dictionary dict
| |
| Image Library Functions | Image.open(fname) | opens and returns image |
| im.show() | displays image im | |
| im.getdata() | returns generator of all pixels in im | |
| im.putdata(pixlist) | modifies image by setting all pixels to pixlist | |
| im.size | returns tuple that is (width,height) of image | |
| Image.new("RGB",size) | creates and returns a new image with dimensions of tuple size | |